1. Field of the Invention
The invention relates to pico second laser sources and in particular to optical parametric oscillators for producing pico second light pulses.
2. Description of the Prior Art
Kafka, xe2x80x9cSynchronously Pumped Subpicosecond Optical Parametric Oscillator,xe2x80x9d U.S. Pat. No. 5,847,861 (1998) shows in FIG. 1A, a cavity defined by highly reflective mirror 12, highly reflective mirror 14, highly reflective mirror 16, curved mirrors 18 and 20, highly reflective mirror 22, and output coupler 24. Positioned on the back side of highly reflective mirror 16 is piezoelectric transducer 26. A high power, one-half Watt or higher pumped source 42 generates subpicosecond pulses in the range of 700 to 900 nm. Mirror 44 focuses pumped beam 46 into the OPO cavity.
Output beam 46 is directed by fold mirrors 48, 50, 52, and focusing mirror 44 along an optical path 54, through curved mirror 18, which is transmissive at the pump radiation wavelength in the range of from about 700 to 900 nm.
Edelstein et.al., xe2x80x9cBroadly Tunable High Repetition Rate Femtosecond Optical Parametric Oscillator,xe2x80x9d U.S. Pat. No. 5,017,806 (1991) shows in FIG. 1, a continuous wave, femtosecond optical parametric oscillator 10 incorporating a thin KTP crystal 12 located in the cavity of a CPM dye laser 14. The OPO ring cavity 24 is formed by two curved focusing mirrors 50 and 52 positioned on opposite sides of the crystal 12 and two flat mirrors 54 and 56. One of the flat mirrors, such as mirror 54, is mounted on a piezoelectric transducer 58 for fine adjustment of the length of the OPO cavity. The crystal 12 acts as a frequency converter, and thus responds to the input energy from the pumping beam of the CPM laser to produce output beams at two longer wavelengths, one represented by the signal beams 20 and 22 and the other represented by the idler beams 16 and 18.
Guyer et.al., xe2x80x9cTunable Pulsed Single Longitudinal Mode Optical Parametric Oscillator,xe2x80x9d U.S. Pat. No. 5,235,456 (1993) shows in FIG. 1, the optical parametric oscillator 2 as including a pump laser source that transmits pulses of optical energy along a pump path 4. The optical parametric oscillator 2 comprises a first means for reflecting optical energy 10, that may be, for example, an optical element such as a back cavity mirror. The first means for reflecting 10 reflects optical energy along the reflective optical path 8 that it receives along the reflective optical path 8. The oscillator 2 also comprises a means for diffracting optical energy 12 that may be, for example, an optical element such as a glancing incidence grating.
The means for diffracting 12 diffracts a first portion of the optical energy directed along the reflective optical path 8 by the first order of interference to a diffraction path 14. The means for diffracting 12 reflects a second portion of the optical energy directed along the reflective optical path 8 along an output optical path 16.
Cheng et.al., xe2x80x9cTunable Optical Parametric Oscillator,xe2x80x9d U.S. Pat. No. 5,053,641 (1991) shows in the embodiment of FIG. 5, a singly resonant oscillator comprised of two identical dichroic mirrors 62 and 64 to define a cavity with a BBO crystal in the cavity.
Campillo et.al., xe2x80x9cBroadly Tunable Picosecond IR Source,xe2x80x9d U.S. Pat. No. 4,349,907 (1982) shows in FIG. 1, a repetitively flashlamp-pumped mode-locked laser emits a train of 40 ps duration pulses on each flashlamp cycle. One beam is directed along a cut lithium niobate crystal, which acts as a traveling-wave parametric oscillator. The emission from the first crystal is diffracted by a grating and the desired spectral and spatial component is injected into a second nearly degenerate 2 cm long LiNbO3 amplifier located in the second pump beam line.
The prior art could generate narrow bandwidth picosecond pulses only by first generating narrow bandwidth pulses with a tunable pulse with a nano-second laser, and then amplifying this pulse with a second pico-second laser. This is a relatively complex and expensive optical system. What is needed is a more integrated system by which narrow bandwidth picosecond pulses can be generated.
An illustration of one application of the invention is its use in gas phase ultra trace detection methods based on resonance enhanced multi photon ionization time-of-flight mass spectrometry for a number of atmospherically relevant compounds. The target molecules include halogen oxides, e.g., HOCl, HOBr, ClOOCl, as well as radical species, e.g., CH3OO and HO2. All these molecules exhibit strong, continuous, broad absorptions in the near UV/VIS region, i.e., dissociate rapidly when excited in this wavelength range. However, a number of relatively long lived Rydberg states are expected closely below the ionization threshold. A two-photon excitation of these levels and consecutive ionization is expected to be a sensitive and selective probe for these molecules. This spectroscopic approach both requires short pulses to compete with the dissociative processes and a narrow bandwidth to obtain high spectroscopic selectivity.
To obtain the radiation required for these measurements, a synchronously pumped picosecond optical parametric oscillator, OPO, system is constructed that produces near-Fourier limited pulses with duration 25 ps. A single pulse produced by this oscillator is amplified in an optical parametric amplifier, OPA, stage. The pulse train pumping the OPO is obtained by xe2x80x9cleakingxe2x80x9d on each round trip a part of the pulse that is formed in the oscillator of a mode locked Nd:Yag laser. This pulse train is then amplified in an additional double pass Nd:Yag amplifier. Next, the third harmonic of the pulse train is generated and directed into the beta-barium borate, BBO, crystals of the OPO. While the illustrated embodiment describes BBO crystals, it must be explicitly understood that the invention includes all optical crystals which are equivalent thereto or substitutes for BBO, such as lithium triborate (LBO) or potassium titanyl phosphate (KTP). For example, various crystal materials discussed in V. G. Dmitriev et.al., xe2x80x9cHandbook of Nonlinear Optical Crystalsxe2x80x9d, 3d Revised Ed. (Springer 1999) may be freely substituted as equivalent to the BBO crystals described in the illustrated embodiment. The oscillator is limited on one side with flat mirror, and with a gazing incidence grating and tuning mirror assembly on the other side. The 0th order reflection of the grating is directed into an OPA, which is comprised of a BBO crystal. The OPA is pumped by a single pulse tunable near to Fourier limited pulse. The system of the illustrated embodiment is comprised of two of these OPO-OPA chains, making it particularly suitable for REMPI and pump-probe measurements. However, it must be understood that the invention can be realized in many different applications and specifically includes subportions of the illustrated embodiment.
The pump laser and the construction of the pulse train amplifier, the OPO and the OPA are disclosed below. Also measurements of the bandwidth, of the temporal behavior of the oscillator versus pump intensity and power gain in the OPA are presented to validate the system.
More particularly, the invention is defined as an apparatus for generating narrow bandwidth picosecond optical pulses comprising a pump laser; an optical parametric oscillator pumped by a pump pulse train generated by the pump laser; and an optical parametric amplifier having an input coupled to an output of the optical parametric oscillator and pumped by a single pulse from the pump laser. The optical parametric oscillator comprises an optical cavity comprised of a grating-mirror termination on one end of the cavity and with optically nonlinear active media and a cavity mirror on an opposing end of the cavity.
The grating-mirror termination on one end of the cavity is comprised of a grazing incidence grating and a tuning mirror. The grating and mirror are arranged with respect to each other so that a diffracted first order is reflected back from the mirror to the grating and into the cavity. In the illustrated embodiment the grazing incidence grating has a periodicity of about 1800 grooves/mm, and is inclined at an approximately 10xc2x0 angle with respect to the axis of the cavity. The groove density or grating constant can be chosen according to the center of the tuning range for the apparatus. The grazing incidence grating has a blaze optimized for grazing incidence to maximize a first grating order of diffraction.
The tuning mirror is rotatable about a center defined about the grating so that wavelength of the cavity can be adjusted without changing the optical length of the cavity.
The pump laser introduces a train of pulses into the optical parametric oscillator of sufficient strength to trigger a multiple number of round trips of pulses in the cavity of the optical parametric oscillator in which each reflection of a pulse from the grating-mirror termination narrows the bandwidth of the pulse.
The cavity of the optical parametric oscillator further comprises a concave mirror, R=75 cm, to image light transmitted to and received from the grating-mirror termination from the opposing end of the cavity to increase stability of the cavity and to lower the threshold of oscillation. The wavefront of the light at the end of the cavity is flattened relative to the wavefront at the center of the cavity.
The grating-mirror termination couples a 0th order diffraction of light from the cavity of the optical parametric oscillator into the optical parametric amplifier.
The pump laser generates a single pulse, which is input into the optical parametric amplifier to coincide with the last pulse of a pulse train output by the optical parametric oscillator and coupled into the input of the optical parametric amplifier.
The optically nonlinear active media is comprised of at least one BBO crystal, and preferably of a pair of BBO crystals arranged with respect to each in a walk-off compensating arrangement to extend power capability of the optical parametric oscillator by increasing the optical gain within the oscillator. Each BBO crystal is independently rotatable to adjust an angular orientation of each BBO crystal in the cavity.
The optical parametric oscillator and optical parametric amplifier in combination generate a pulse having a bandwidth characterized by a Fourier limit with the bandwidth of the generated pulse being near the Fourier limit.
The invention further comprises a method for generating a narrow bandwidth picosecond optical pulse comprising the steps of generating a pump laser pulse train; pumping an optical parametric oscillator by the pump pulse train; generating a pulse train output with a narrowed bandwidth and picosecond pulse width from the optical parametric oscillator by use of a grating-mirror termination on one end of a cavity in the optical parametric oscillator and a cavity mirror on an opposing end of the cavity with optically nonlinear active media therebetween; pumping an optical parametric amplifier having an input coupled to an output of the optical parametric oscillator; and outputting the narrow bandwidth picosecond optical pulse from the optical parametric amplifier.
While the apparatus and method has or will be described for the sake of grammatical fluidity with functional explanations, it is to be expressly understood that the claims, unless expressly formulated under 35 USC 112, are not to be construed as necessarily limited in any way by the construction of xe2x80x9cmeansxe2x80x9d or xe2x80x9cstepsxe2x80x9d limitations, but are to be accorded the full scope of the meaning and equivalents of the definition provided by the claims under the judicial doctrine of equivalents, and in the case where the claims are expressly formulated under 35 USC 112 are to be accorded full statutory equivalents under 35 USC 112. The invention can be better visualized by turning now to the following drawings wherein like elements are referenced by like numerals.